4784: Change management of test cfg to better support json projects r=Nashenas88 a=Nashenas88
This helps support json projects where they can decide whether to add the `test` cfg or not. One alternative is to add support for marking json project crates as a sysroot crate, and adding logic to remove the `test` cfg in those cases. In my opinion, that option gives less flexibility to json projects and leads to more functionality that needs to be maintained.
Fixes#4508
cc @woody77
Co-authored-by: Paul Daniel Faria <Nashenas88@users.noreply.github.com>
Co-authored-by: Paul Daniel Faria <nashenas88@users.noreply.github.com>
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